Human susceptibility to the harsh space radiation environment has been identified as being a major hurdle for exploration beyond low Earth orbit (LEO). High energy protons and nuclei ions from Solar Energetic Particles (SEPs) and Galactic Cosmic Rays (GCRs) can result in radiation doses that are dangerous to astronaut health and even survivability if the astronauts are not adequately shielded. These high energy particles also cause significant amounts of secondary radiation when they impinge on spacecraft structure. The secondary neutron radiation may cause human radiogenic cancers. Hydrogen or hydrogen rich materials are ideal materials for radiation shielding because hydrogen does not easily break down and become a source for secondary radiation.
When a spacecraft is positioned in LEO, the Earth's magnetic field provides some radiation protection to the spacecraft and the astronauts occupying it. Radiation protection for astronauts is critical for the future of human space flight since conventional spacecraft construction materials such as aluminum are susceptible to secondary radiation when SEPs or GCRs impinge on them. Because of the size of an aluminum nucleus, the secondary radiation produced while shielding space radiation can be just as damaging as the primary radiation and this secondary radiation contributes to the total ionizing dose received by the astronauts. Other types of hydrogen-rich materials, such as polyethylene, have been tested to determine their effectiveness at reducing the dose received from all sources of radiation. Such shielding materials do not produce the same level of damaging secondary radiation, however, the presence of carbon atoms in polyethylene means that there is less hydrogen shielding material per unit of shielding material mass than there would be if hydrogen itself is used as the shielding material. However, hydrogen is a challenging substance to store and manage and, therefore, has not been considered as a viable shielding material for spacecraft.
Developing a system using cryogenic material, hydrogen, that is maintained at, for example, 10-12 K (“K” here and throughout refers to “° K” or “degrees Kelvin”), for radiation shielding presents several challenges. Thermal challenges include, for example, heat leak from the space environment into cryogenic hydrogen shielding due to, for example solar irradiation, planetary albedo, heat leak from the crew capsule that is maintained at room temperature of about 300 K, power system, propulsion, etc. into the cryogenic hydrogen shield. It is also challenging to process the cryogenic hydrogen on the ground, prior to launch, and bring it to a frozen temperature of 10 K while the hydrogen is contained in a tank that is in an ambient approximately 300 K environment.